Remove unnecessary $posts_per_page global in admin.php

Description

There is an orphaned $posts_per_page = get_option('posts_per_page') in the admin.php bootstrap. The variable is never used and appears to be an artifact that is left over from before the time of screen options.

The posts_per_page option is the reading option for the number of posts to display per page in a theme.

In the off-chance a plugin used this variable, it is overwritten in post.php anyway, and would provide incorrect values on any other page, as they have their own *_per_page user options. As of [12375] they also have a complete set of filters as well.

If someone feels that broken back compat is necessary here, then we can wontfix.